The major purpose of the proposed investigation is to examine the effects of a comprehensive, school-based, community-linked intervention on physical activity and physical fitness in middle school girls. Factors that mediate the effect of the intervention will be identified, and the intervention's impact on tobacco use, alcohol use, and depressive symptomatology will be observed. A collaborative, multi-center investigation will be conducted. At each of five field centers, six middle school catchment areas will be pair-matched and randomly assigned to intervention and control groups. In the intervention catchment areas a multi-component intervention, based on an ecological model and Social Cognitive Theory, will be implemented. The school physical education curriculum will be modified to emphasize in-class participation in moderate to vigorous physical activity and mastery of behavioral skills related to physical activity. Relationships will be established between the school and at least two community organizations that will provide after- school and summer physical activity programs for girls at the school site. in addition, after-school, weekend, and summer physical activity programs for girls will be expanded in community settings. Modifications will be made in several components of the school health program to link girls to the expanded physical activity program opportunities. Girls will be exposed to the intervention during their 7th and 8th grade years. Baseline measures of physical activity, physical fitness, other health characteristics and psychosocial/environmental factors related to physical activity will be administered during the spring of the 6th grade year, and this measurement battery will be re-administered during the 7th, 8th and 9th grade years. To determine intervention effects, statistical analyses of the physical activity and fitness data will be performed using mixed- model analysis of variance, with catchrnent area (nested within treatment) taken as the unit of analysis. It is hypothesized that the intervention will significantly reduce the usual rates of decline in physical activity and fitness seen in girls between the 6th and 8th grades. This study will uniquely and markedly expand our knowledge of promotion, determinants, and health benefits of physical activity in middle school girls.